Ram air turbines are used in aircraft to provide electric and/or hydraulic power to components of the aircraft, for example in the event of failure of other electrical or hydraulic power generating equipment on board the aircraft. In normal operation of the aircraft, the RAT is housed within the aircraft, but when needed it is extended into the airflow around the aircraft such that it may turn and generate electrical and/or hydraulic power. The RAT is typically pivoted into position by means of an actuator, for example a hydraulic actuator. The actuator typically includes a piston which extends to deploy the RAT. The actuator has a lock mechanism which prevents inadvertent movement of the piston and thus inadvertent deployment of the RAT. The locking mechanism typically comprises a spring loaded lock piston which must be moved in an axial direction to unlock the actuator. Such a mechanism is disclosed for example in US 2013/0327207 A1.
An unlocking mechanism is provided to permit this movement. Typically, the unlocking mechanism comprises a two link, knee type linkage one end of which is rotatably coupled to one end of the locking piston and the other end of which is axially fixed and rotatably coupled to a support. A linear solenoid moves the linkage between a locked and an unlocked position. However, in moving between the two positions, the linkage must go “over centre” and initially displace the lock piston against the force of the lock piston spring, which means that a relatively large solenoid may be necessary.
As weight is of concern in aircraft, it would be desirable to provide a locking mechanism which may potentially be lighter.
A further problem with the over-centre mechanism and the high force required from the solenoid is that the solenoid's field is non-linear. Therefore an accurate positioning of the solenoid's plunger within the coil is necessary in order to keep the size (and weight) of the solenoid to a minimum. However, this reduced tolerance adds cost to installation by requiring more accurate positioning.